1. Field of the Invention
The present invention relates to pellets incorporated with a carbonaceous material and a method of producing reduced iron using the same.
2. Description of the Related Art
As a method of producing reduced iron, a MIDREX method is well known, in which a reducing gas obtained by degenerating natural gas is blown into a shaft furnace through tuyeres, and moved upward in the shaft furnace so that iron ore or iron oxide pellets filled in the furnace are reduced to obtain reduced iron. However, this method requires the supply of a large amount of natural gas expensive as fuel.
Therefore, a process for producing reduced iron using relatively inexpensive coal as a reducing agent in place of natural gas has recently attracted attention. For example, U.S. Pat. No. 3,448,981 discloses a process for producing reduced iron comprising heating and reducing pellets incorporated with a carbonaceous material, which are obtained by pelletizing a mixture of fine ore and a carbonaceous material in an atmosphere of high temperature. This process has the advantages that coal is used as the reducing agent, fine ore can be used directly, high-rate reduction is possible, the carbon content in a product can be adjusted, etc.
In this process, the pellets incorporated with the carbonaceous material are heated by radiant heat from the upper side of a high-temperature reducing furnace, and thus the height of the pellet layer is limited. Therefore, in order to improve productivity, it is necessary to increase the reaction rate of reduction. However, since the reduction rate of the pellets is controlled by heat transfer within the pellets, when the temperature of the reducing furnace is increased to the heat transfer limit of the pellets or more in order to improve productivity, the pellets incorporated with the carbonaceous material are melted from the surfaces thereof to cause the problems of sticking in the furnace, and damage to the furnace body. The pellets incorporated with the carbonaceous material include pellets formed by pelletizing a mixture of raw materials such as fine ore, a carbonaceous material such as coal or the like serving as the reducing agent, and a binder, by using a pelletizer or a molding machine. In this case, the formed pellets incorporated with the carbonaceous material are porous, and have a small area of contact between the carbonaceous material and the fine ore, low thermal conductivity, and a low reduction rate.
Accordingly, an object of the present invention is to provide pellets incorporated with a carbonaceous material, which are capable of promoting reduction reaction of iron oxide, and which exhibit excellent strength after reduction, and a method of producing reduced iron using the pellets incorporated with a carbonaceous material with high productivity.
The pellets incorporated with a carbonaceous material of the present invention comprise a carbonaceous material and iron ore mainly composed of iron oxide, wherein combinations of the maximum fluidity of the carbonaceous material in softening and melting and the ratio of iron oxide particles of 10 xcexcm or smaller in the iron ore are within the range above a line which connects points A, B and C shown in FIG. 1, including the line. Point A shown in FIG. 1 is a point where the maximum fluidity is 0, and the ratio of iron oxide particles of 10 xcexcm or smaller in iron ore is 15% by mass; point B shown in FIG. 1 is a point where the maximum fluidity is 0.5, and the ratio of iron oxide particles of 10 xcexcm or smaller in iron ore is 1% by mass; point C shown in FIG. 1 is a point where the maximum fluidity is 5, and the ratio of iron oxide particles of 10 xcexcm or smaller in iron ore is 1% by mass.
In this case, it is possible to promote reduction reaction of iron oxide, and obtain the pellets incorporated with the carbonaceous material exhibiting excellent strength after reduction.
In the present invention, the maximum fluidity is a value which is represented by logDDPM, and obtained by measurement using a Gieseler Plastometer, as defined by JIS M8801. The iron oxide particles of 10 xcexcm or smaller in iron ore are measured by a wet laser diffraction method.
In the present invention, the carbonaceous material represents a carbon-containing material such as coal, coke, petroleum coke, pitch, tar, or the like, and any carbonaceous material which satisfies the above-described relation between the maximum fluidity of the carbonaceous material in softening and melting, and the ratio of iron oxide particles of 10 xcexcm or smaller in iron ore can be used. The carbonaceous material may be a single material or a mixture of a plurality of materials, and the iron ore may be a single type or a mixture of a plurality of types.
The pellets incorporated with the carbonaceous material have excellent thermal conductivity, and thus the use of the pellets can realize improvement in productivity of the production of reduced iron.